It has been recognised that there is a need to provide accurate information to a user of a dose-dispensing delivery apparatus, such as a pressurised metered dose inhaler, concerning the number of doses delivered from, or remaining in, the dispensing apparatus. Without such information, there is a danger that a user will forget how many doses have been delivered and hence take a greater or fewer number of doses than are required. There is also a danger that a user may be unaware that the dispensing container of the dispensing apparatus is empty or close to empty. This is especially dangerous where the dispensing apparatus is for use in delivering medicinal compounds for the treatment of chronic or acute symptoms, for example, as in the case of a pressurised metered dose inhaler used for treating asthmatic reactions.
It is known to provide a dispensing apparatus with a dose counting device. Typically such dose counting devices are triggered by movement of the dispensing container wherein the movement either directly or indirectly provides the motive force for incrementing or decrementing the dose counting device. In order for a dose counting device to be accurate it must be designed not to under-count the number of doses dispensed from the dispensing container, otherwise a user may be misled into thinking that the dispensing apparatus has a greater number of remaining doses than is actually the case. There has therefore been a desire to develop dose counting devices where movement of the dispensing container into its actuated position in the dispensing apparatus is not possible without incrementing or decrementing the dose counter device. In this way under-counting is prevented. However, a disadvantage of such dispensing apparatus is that actuation of the dispensing container is not possible if the dose counting device should inadvertently jam or become inoperative for some other reason. There is then the danger that a user would not be able to actuate the dispensing container despite there being available doses therein.
There can also be a problem with known mechanical dosage counters in that they are prone to accidentally incrementing if the apparatus is dropped due to impact forces imparted on the counting mechanism.
Mechanical dose counters can also be difficult to manufacture so that they work reliably. One problem is that a mechanical dose counter typically requires a number of components which must be accurately located relative to one another during assembly to ensure that over-counting or under-counting is not experienced. This can lead to a requirement for very strict manufacturing tolerances which can make manufacture expensive.